This disclosure relates generally to control systems for turbines, and more specifically to measuring generator operating parameters to detect a loss of load that results in a large unbalance between power and load that would result in turbine-generator overspeed trip and provide turbine overspeed protection that will avoid a turbine trip due to the emergency overspeed trip device.
In a typical reheat turbine speed-load control system, valves regulate steam flow to both the high-pressure and reheat stages of the turbine in order to maintain the turbine speed at a predetermined reference speed. A main control valve regulates steam flow into the high-pressure stages and an intercept control valve regulates steam flow into the reheat stages. On the occurrence of an overspeed condition while the generator is separated from the system, the main control valve and intercept control valve are totally or partially closed to decrease steam flow into their respective stages to effect a reduction of turbine speed to the reference speed.
These reheat turbine speed-load control systems operate effectively to prevent an emergency overspeed trip on the occurrence of sudden total load loss where the load lost is small compared to the total generating capacity of the turbine-generator set. In this case a rapid complete closure of the intercept control valve causes the loss of a major portion of the driving power normally provided by the reheat stages and similarly steam flow to the high-pressure stages is reduced by closure of the main control valve. The control basis for these valve closures is the difference between the actual speed and the reference speed.
On the occurrence of sudden severe total loss of load where the load lost is comparable to a significant portion of the generating capacity of the turbine generator set, the turbine speed-load control systems, as described above, can not prevent an emergency overspeed trip. Delaying rapid complete closure of the intercept valve and control valve until the actual speed increases above the reference speed in order to avoid an emergency overspeed trip is not effective due to the larger control and intercept valve opening and the larger power density at high load conditions. Initiation of control valve and intercept valve closure must be advanced in time sufficiently to begin cut-off of steam prior to actual speed increase. Total electrical load loss is characteristically a rapid event when compared to rate of change for turbine power production. This situation can be used to provide detection of the conditions leading to turbine generator overspeed as described in this disclosure and prior art. When the characteristic conditions of load loss and unbalance between power and load are detected as described in this disclosure, rapid closure of turbine steam control valve and intercept valve is initiated, removing a major portion of the driving power to avoid an emergency overspeed trip. Hence, once the steam supply to the turbine is cut-off, the speed will increase due to the entrained steam mass as it proceeds to exhaust, but the speed increase will not be as rapid and will peak at a lower value thus avoiding an emergency overspeed trip. After the entrained steam is exhausted, speed will decrease and the turbine intercept valve and control valve will open based on speed-load control.